On Mar 22, 2010, at 5:36 PM, Pawel Jakub Dawidek wrote: > On Mon, Mar 22, 2010 at 08:23:43AM +0000, Poul-Henning Kamp wrote: >> In message <4ba633a0.2090...@icyb.net.ua>, Andriy Gapon writes: >>> on 21/03/2010 16:05 Alexander Motin said the following: >>>> Ivan Voras wrote: >>>>> Hmm, it looks like it could be easy to spawn more g_* threads (and, >>>>> barring specific class behaviour, it has a fair chance of working out of >>>>> the box) but the incoming queue will need to also be broken up for >>>>> greater effect. >>>> >>>> According to "notes", looks there is a good chance to obtain races, as >>>> some places expect only one up and one down thread. >>> >>> I haven't given any deep thought to this issue, but I remember us discussing >>> them over beer :-) >> >> The easiest way to obtain more parallelism, is to divide the mesh into >> multiple independent meshes. >> >> This will do you no good if you have five disks in a RAID-5 config, but >> if you have two disks each mounted on its own filesystem, you can run >> a g_up & g_down for each of them. > > A class is suppose to interact with other classes only via GEOM, so I > think it should be safe to choose g_up/g_down threads for each class > individually, for example: > > /dev/ad0s1a (DEV) > | > g_up_0 + g_down_0 > | > ad0s1a (BSD) > | > g_up_1 + g_down_1 > | > ad0s1 (MBR) > | > g_up_2 + g_down_2 > | > ad0 (DISK) > > We could easly calculate g_down thread based on bio_to->geom->class and > g_up thread based on bio_from->geom->class, so we know I/O requests for > our class are always coming from the same threads. > > If we could make the same assumption for geoms it would allow for even > better distribution.
The whole point of the discussion, sans PHK's interlude, is to reduce the context switches and indirection, not to increase it. But if you can show decreased latency/higher-iops benefits of increasing it, more power to you. I would think that the results of DFly's experiment with parallelism-via-more-queues would serve as a good warning, though. Scott _______________________________________________ freebsd-current@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-current To unsubscribe, send any mail to "freebsd-current-unsubscr...@freebsd.org"
